Electroencephalography MEG) and functional MRI (fMRI) induce mutual artifacts when recorded concurrently. Electroencephalography (EEG) has been a key tool for study of the brain for decades. However, despite its multiple clinical and research uses, such as in epilepsy (Ebersole, 1997), sleep staging (Rechtschaffen & Kales, 1968) and psychophysiology, little is yet known about the underlying generators of EEG activity in humans. Functional MRI (fMRI) recorded in concert with EEG can provide a method for localizing these sources. By using the ERG signal as a reference for fMRI maps, concurrent EEG/fMRI opens a new avenue for investigating specific brain function. There remains a need for a system for simultaneous recording of EEG and fMRI, which can be used as a tool to localize sources of the EEG.
Simultaneous recording of EEG and fMRI has proven challenging. Time varying magnetic (B) fields induce an electromotive force (e.m.f.) in a wire loop perpendicular to the B field direction which, by Lenz's Law, is proportional to the cross sectional area of the wire loop and to the rate of change of the perpendicular magnetic field (dB/dt). When EEG leads are placed inside the MR scanner, the rapidly changing gradient fields and the radio-frequency A) pulses required for MRI may induce voltages that obscure the EEG signal (Huang-Hellinger, et al., 1995; Ives, Warach, Schmitt, Edelman, & Schomer, 1993). The induced e.m.f. yields currents that can cause heating of the electrodes and leads and potentially impart burns to the patient (Lemieux, Allen, Franconi, Symms, & Fish, 1997). Motion of the leads themselves within the static field of the magnet also induces an e.m.f.; even pulsatile motion related to heart beat yields ballistocardiographic artifact in the EEG that can be of roughly the same magnitude as the EEG signals themselves (Ives, Warach, Schmitt, Edelman, & Schomer, 1993; Muri, et al, 1998). Further, introduction of EEG equipment into the scanner potentially can disturb the homogeneity of the magnetic field and distort the resulting MR images.
In addition to the large artifacts in the EEG caused by high frequency gradient and RF pulses, the high pass filters of most EEG equipment lead to long signal recovery times once the MR acquisition has terminated (Krakow, et al., 1999). One method used to overcome these difficulties in studies of epilepsy has been to monitor the EEG in the absence of scanning while the patient is in the magnet and to then trigger functional scanning manually after identification of inter-ictal spikes in the EEG record (Krakow, et al., 1999; Seeck, et al., 1998; Warach, et al, 1996). Visual evoked potential has been studied using interleaved blocks of EEG and fMRI, where the same stimuli are presented in each block (Bonmassar, Anami, Ives, & Belliveau, 1999). In these methods, EEG and fMRI are acquired serially, resulting in protocol limitations and problems with data analysis. In the triggered method, relevant changes in the EEG can not be seen during functional scanning. Problems also exist with non-uniform MR image contrast, given that T1 saturation typically does not reach equilibrium until 3 to 4 TRs after initiation of the scan (depending on TR and effective flip angle). Most often this is handled by ignoring images acquired in the first 3-4 TRs, but this then leads to an inherent time delay in the functional scanning. This could be mitigated to some degree by using schemes that correct for the T1-related intensity differences based on the actual TR (DuBois & Cohen, 2000; Guimaraes, et al., 1998). In the interleaved method, in addition to the former confounds, the EEG and fMRI can not be compared directly.